1. Field of the Invention
The present invention relates to a machine tool which is adapted to fix a workpiece to a workpiece spindle by non-self-aligning clamp means such as a magnetic chuck or a face-clamp and, particularly, to a machine tool having an automatic workpiece centering function.
2. Description of Related Art
In a machine tool such as a CNC vertical grinder having a vertical workpiece spindle for rotating a workpiece, for example, a magnetic chuck for clamping the workpiece by utilizing an attractive force of an electromagnet is provided on an upper end of the workpiece spindle. The workpiece is machined, while the workpiece spindle is rotated with the workpiece being clamped by the magnetic chuck.
Meanwhile, a mechanical chuck including a plurality of chuck claws movable radially inwardly and outwardly of the workpiece spindle for clamping the workpiece has a self-aligning function such that the center of the workpiece is naturally aligned with the rotation center of the workpiece spindle when the workpiece is clamped. On the contrary, the aforesaid magnetic chuck is merely adapted to hold the workpiece placed directly on a horizontal surface thereof or with the intervention of a chuck block by the attractive force of the electromagnet and, hence, has no self-aligning function unlike the mechanical chuck.
In the case of the machine tool including the non-self-aligning magnetic chuck for clamping the workpiece, the center of the workpiece should be aligned with the rotation center of the workpiece spindle before the workpiece placed on the magnetic chuck is clamped. Therefore, an operator manually performs a workpiece centering operation whenever the workpiece is mounted on the machine tool.
The workpiece centering operation is performed by rotating the workpiece spindle with the workpiece being placed on the magnetic chuck, bringing a dial gage or the like into contact with an outer circumferential surface of the workpiece for measuring a run-out of the workpiece resulting from off-center of the workpiece, and repeatedly correcting the position of the workpiece to minimize the run-out. This requires great effort and time, thereby imposing a great burden on the operator in a preparatory operation. In addition, the machining of the workpiece cannot efficiently be performed in a short time.
It is therefore an object of the present invention to provide a machine tool which is capable of efficiently machining a workpiece in a short time while minimizing the burden on the operator even if non-self-aligning clamp means is employed for clamping the workpiece.